Developers used for electrophotographic copying apparatus, facsimile apparatus, printers, etc. include a two-component type developer in the form of a mixture of a magnetic carrier and a color toner. When an electrostatic latent image is developed with such a developer, the color toner is consumed by attaching to the latent image. However, the magnetic carrier in the developer does not decrease, resulting in a decrease in the ratio of the color toner to the magnetic carrier in the developer (which ratio will be referred to hereinafter as toner concentration).
For attainment of good-quality development, it is necessary to maintain the toner concentration 11 of the developer within a predetermined range, and, for this purpose, there is a toner concentration control apparatus which detects the toner concentration of the developer to replenish the color toner in the developer.
As toner concentration detecting means in the above-described prior art apparatus, a planar electric coil has been disposed at a suitable position in the developer container surrounded by the stream of the developer, and, utilizing the fact that the coil inductance increases with the decrease of the toner concentration of the developer, the coil inductance value has been measured to detect the toner concentration.
However, the detection by measurement of the inductance has been defective in that temperature compensation is required to deal with variations of the inductance value due to changes in temperature and humidity, and, although the above problem is avoided by, for example, the additional provision of a temperature compensation circuit, sufficient temperature compensation is difficult when fluctuations between products are considered together with other problems including an increase in the number of component parts.
Another toner concentration detecting means is based on the fact that, in the developer contained in the developer container, the color toner only is consumed so that the quantity of the carrier remains constant without being consumed. Accordingly, a developer level detector is disposed at a predetermined position in the developer container to monitor the quantity of the developer, and the shortage is filled up by the color toner so as to control the toner concentration. This developer level detector may be implemented as a back-coupling oscillation circuit using an electric coil acting as a detecting member to detect the level of the developer on the basis of the oscillation level of this oscillation circuit.
However, the above proposal has been defective in that the successful condition of oscillation of the above-described back-coupling oscillation circuit is quite sensitive to the external environmental conditions including the temperature and humidity, and, because of such a problem, detection of the developer level may become utterly impossible, rendering sufficient compensation extremely difficult.